text: The length of the hydrodynamic entry region along the pipe is called the hydrodynamic entry length. It is a function of Reynolds number of the flow. In case of laminar flow, this length is given by:
  where  is the Reynolds number and  is the diameter of the pipe.<br>But in the case of turbulent flow,
  Thus, the entry length in turbulent flow is much shorter as compared to laminar one. In most practical engineering applications, this entrance effect becomes insignificant beyond a pipe length of 10 times the diameter and hence it is approximated to be:
  <br />Other authors give much longer entrance length, e.g. 
  * Nikuradse recommends  and
  * Lien et al. recommend  for high Reynolds flows.